The present invention relates to electroacoustic transducers, and more particularly to a diffraction electroacoustic transducer useful as an acoustooptic deflector, modulator or the like to generate a supersonic wave having a variable wideband frequency or a very short pulsed waveform.
A conventional wideband transducer has the shape of a thin plate with metal electrodes plated on both sides thereof. To ensure a flat frequency response over a wide frequency range, any acoustic resonance in the frequency range of interest should be avoided. Principal acoustic resonances occur at every integral multiple of the frequency f.sub.c which is expressed as EQU f.sub.c = v.sub.a /( 2D), (1)
in terms of the acoustic wave-velocity v.sub.a along a direction perpendicular to the thin plate and the thickness D of the plate.
When the transducer is bonded to an acoustooptic material having an acoustic impedance perfectly matched with that of the transducer material, acoustic resonance can be avoided. However, the electroacoustic conversion efficiency of the transducer is greatly reduced when the transducer is used over a frequency range of several multiples of the frequency f.sub.c. Therefore, the frequency f.sub.c is a kind of measure of the practical limit of the frequency band for thin plate type transducers. For a typical value of v.sub.a, 6 .times. 10.sup.3 m/sec, the thickness D of the thin plate must be less than 30 micrometers for a transducer having a band-width of a hundred megahertz.
Thus, extensive efforts have been made in the fabrication of thin transducers. For instance, a transducer fabricated to 3 micrometers using a spattering erosion technique is reported by J. D. Larson, III and D. K. Winslow in an article entitled "Ultrasonically Welded Piezoelectric Transducers," which appeared in IEEE Transactions on Sonics and Ultrasonics, vol. SU-18, No. 3, pp. 142-152, July 1971. The reported transducer has seemed to be the practical limit of those thin plate type of transducers.
On the other hand, to generate a surface acoustic wave, the so-called "interdigital transducer" has been proposed by J. H. Coquin and H. F. Tiersten, and a transducer usable up to 1.7 gigahertz has been achieved by A. N. Boers et al. as reviewed by Ernest Stern in an article entitled "Microsound Components, Circuits and Applications" which appeared in IEEE Transaction on Microwave Theory and Techniques, vol. MTT-17, No. 10, pp 835-844, November 1969. The interdigital transducer comprises a set of parallel metal strips plated on a piezoelectric material and alternately connected to each opposite terminal of the transducer. The interdigital transducer has a maximum efficiency around the frequency f.sub.o which satisfies the relation, EQU f.sub.o = v.sub.a /( 2S), (2)
given in terms of the spacing S between two adjacent metal strips. An interdigital surface electrode having a spacing S of a few micrometers is easily made by photolithographic technology developed in recent years.
Equation (2) may be interpreted as a kind of wave matching condition, EQU K.sub.a = (2.pi.f.sub.a) / v.sub.a = .pi./S, (3)
between the wave vector K.sub.a of the surface acoustic wave and the wave vector .pi./S of the spatially periodic change in the electric field having a period 2S. An analogous wave matching excitation of a volume acoustic wave is discussed by G. A. Coquin et al. in a paper entitled "Wide Band Acoustooptic Deflectors Using Acoustic Beam Steering" which appeared in IEEE Transactions on Sonics and Ultrasonics, vol. SU-17, No. 1, pp. 34-40, January 1970. In this case the wave matching condition is discussed merely in connection with an effective acoustic wavefront in an array of beams of acoustic waves, each of which is excited by each separate transducer of the thin plate type. Each adjacent transducer is mounted on each stepped surface of depth s and height h, excited with a phase angle shifted .pi.-radians from each other. The inclination .theta..sub.e of the average wavefront over the array of beams measured from the plane of each transducer satisfies the condition, EQU .pi.S .congruent. K.sub.a [(h/s) - .theta..sub.e ]. (3')
Equation (3) and Equation (3') are special and approximate expressions of a more general wave matching condition, EQU m(.pi./S) = K.sub.a (sin.theta..sub.i - sin.theta..sub.e), (3")
which is applied to the diffraction of a wave having a wave-number K.sub.a, an angle of incidence .theta..sub.i and an angle of diffraction .theta..sub.e by a structure having a period 2S.